[1] The interpretation of electromagnetic anomalies associated with volcanic activity requires a good understanding of two rock properties, the electrical conductivity and the streaming potential coupling coefficient. We measured these properties on 22 consolidated tuff samples containing clays and zeolites typically found in hydrothermal systems or in other areas of high water-rock interactions associated with active volcanic areas. These rocks exhibit unusually high surface conductivity and electrical cementation exponent (2.5-3.3). These features are explained by the highly complex texture of these rocks due to hydration/dissolution processes of the volcanic glass and the formation of clay minerals and zeolites as alteration products. At a pore fluid conductivity of 1.0 ± 0.2 S m À1 , the streaming potential coupling coefficient ranges from À3.55 to À10.7 mV MPa À1 . The zeta potential (a key electrochemical property of the pore water/mineral interface) determined from these measurements at T 0 = 20°C and pH $6-7 and corrected for surface conductivity is $À28 ± 8 mV at 0.1 M NaCl. Two clay-free samples exhibit a zeta potential $À16 ± 5 mV, a value associated with zeolites (clinoptilolite). The temperature dependence of the z potential is rather strong with z(T ) = z(T 0 ) [1 + n z (T À T 0 )], where n z % 4.2 Â 10 À2 C À1 in the temperature range 20-60°C. These data are applied to the understanding of large-scale self-potential anomalies located at the ground surface of Miyake-Jima volcano.

International audienceIn this paper, a combined approach of geomorphic analyses, detailed geological mapping on SPOT 5 satellite images and field surveys, complemented by radiometric dating (Ar-40/Ar-39) allows proposing a new and consistent tectonic configuration for the northeastern Arabia-Eurasia collision zone. It favors a simple strike-slip faulting mechanism in NE Iran accommodating the northward motion of central Iran with respect to Eurasia between the Binalud and Kopeh Dagh mountains. Our results demonstrate that similar to 2 mm/a (similar to 25%) of this northward motion has been transferred from the Binalud to the Kopeh Dagh through the Meshkan Transfer Zone since similar to 5 Ma. The modern state of stress deduced from fault kinematics analyses shows N30 degrees E trending maximum stress axis and confirms a dominant strike-slip faulting mechanism. Combining our geological data with other available data, we conclude that kinematics of deformation can be appropriately described by rigid block faulting in NE Iran; instead, the continuous deformation as it has been thought during the last 3 decades

[1] The data and analysis presented in this paper provide an assessment of lava morphologies and the geochemistry of lavas from the Oman ophiolite. In order to provide detailed constraints on the construction of the upper oceanic crust, a continuous volcanic transect (300 m-thick) was sampled at high-frequency in the Semail ophiolite along Wadi Shaffan. The Wadi Shaffan section is composed mainly of pillow lavas interbedded with massive flows and occasional hyaloclastites. The sampling performed along Wadi Shaffan implies temporal variations in the activity of the ridge. The section is characterized by chemical compositions consistent with those of V1-Geotimes volcanism. The Wadi Shaffan transect was built through two main petrological and geochemical sequences of volcanic activity. Trace element ratios (e.g. Zr/Nb and La/Yb) allow us to distinguish two main sequences with two different parental magmas. Differences in the degree of partial melting are required to explain these trace element ratio variations. Beyond these differences in parent melt composition, variations in trace element abundances (TiO 2 , Zr, REE) involve differentiation processes prior to emplacement. In the lower sequence, less differentiated lavas are in the upper part of the cycle. Magma mixing is proposed to explain this reversed geochemical evolution through time. In the upper sequence, geochemical analysis suggests a different magma chamber process. This sequence consists of multiple events of magma emplacement. Variations in trace element abundance suggest four magmatic cycles. Each magmatic cycle is characterized by primitive lavas evolving to more differentiated lavas with time. The upper sequence lavas appear to be in equilibrium with clinopyroxene and lower sills from the MTZ (Mantle-Crust Transition Zone) and with lower gabbros. We propose a model in which the upper sequence lavas were directly derived from the MTZ and lower gabbro sills and then transported to the surface without interaction with higher crustal levels.

Abstract. Pore-space topology. is studied in recent compact angular blocks and scoria fragments •¾om block-and-ash flow deposits of Merapi volcano, Indonesia. Connected porosity and electrical conductivity of cored clay4¾ee samples are measured in the laboratory at varying fluid salini,ty. The electrical •brmation fiactor, tortuosity and cementation index are derived and the pore-,space topology is •bund to be porosity-dependent. The electrical flow pattern is controlled by crack-like paths in compact angular block, and by vesicles geometry in more porous scoria •¾agments. Because Merapi lavas are highly viscous and crystal-rich, we infer that the development of pore connectedness and vesicle coalescence is promoted by shear strain. Along with ascent rate considerations we conclude that degassing of Merapi magma occurs mostly in the conduit during ascent.

Scattered ignimbritic mesas crop out in the Hermosillo region (Sonora, Mexico). These rocks that have been dated at 12.5 Ma (Middle Miocene) have the petrography and chemical characteristics of comendites. Such a flare-up of peralkaline acidic volcanism, after a long period of subduction-related arc volcanism, emphasises an important change in the source of volcanism. It corresponds to the latest stage of continental extension prior to the marine invasion and the development of spreading centres in the Gulf of California.